Solution

Photo by: Olivier

A solution is a homogeneous mixture of two or more substances. The term
homogeneous means "the same throughout." For example,
suppose that you make a solution of sugar in water. If you were to take a
drop of the sugar solution from anywhere in the solution, it would always
have the same composition.

Terminology

A number of specialized terms are used in talking about solutions. The
solvent in a solution is the substance that does the dissolving. The
solute is the substance that is dissolved. In the sugar solution described
above, the water is the solvent and the sugar is the solute.

Although that definition is neat, it does not always make a lot of sense.
For example, one can make a solution of two gases. In fact, the air around
us is a solution consisting of oxygen, nitrogen, argon, carbon dioxide,
and other gases. In this case, it is difficult to say which gas
"does the dissolving" and which gas (or gases) "is
dissolved."

An alternative method of defining solvent and solute is to say that the
component of the solution present in the largest amount is the solvent
while the components present in lesser amounts are solute. According to
that definition, nitrogen is the solvent in atmospheric air because it is
present in the largest amount. Oxygen, argon, carbon dioxide, and other
gases, then, are the solutes.

The term miscible is often used to describe how well two
substances—generally, two liquids—mix with each other. For
example, if you try to mix oil with water, you will find that the two do
not mix very well at all. They are said to be immiscible—incapable
of mixing. In contrast, ethyl alcohol and water are completely miscible
because they mix with each other in all proportions.

Words to Know

Concentration:
The amount of a substance (solute) present in a given volume of solvent
or solution.

Homogeneous:
The same throughout.

Miscibility:
The extent to which some substance will mix with some other substance.

Saturated:
In referring to solutions, a solution that contains the maximum amount
of solute for a given amount of solvent at a given temperature.

Solubility:
The tendency of a substance to dissolve in some other substance.

Solute:
The substance that is "dissolved" or that exists in the
least amount in a solution.

Solvent:
The substance that "does the dissolving" or that exists
in the largest amount in a solution.

Supersaturated:
In referring to solutions, a solution that contains more than the
maximum amount of solvent that can normally be dissolved in a given
amount of solvent at a given temperature.

Unsaturated:
In referring to solutions, a solution that contains less than the
maximum amount of solvent that can be dissolved in a given amount of
solvent at a given temperature.

Solubility is a term similar to miscibility but more exact. The solubility
of a substance is the amount of the substance that will dissolve in a
given
amount of solvent. For example, the solubility of sugar in water is
approximately 90 grams of sugar per 100 grams of water. That statement
means that one can dissolve up to 90 grams of sugar in 100 grams of water.

The solubility of a substance is dependent on the temperature. The
statement in the previous paragraph, for example, should have been that 90
grams of sugar will dissolve in 100 grams of water at some specific
temperature. That temperature happens to be 0°C.

Generally speaking, the solubility of substances increases with
temperature. The graph in Figure 1 illustrates this point. Notice that the
solubility of sugar increases to a little over 100 grams per 100 grams of
water at 25°C and to 130 grams per 100 grams of water at
50°C.

An important exception to this rule concerns gases. All gases become less
soluble in water as the temperature increases.

Concentration of solutions

Solutions are mixtures whose composition can vary widely. One can make a
water solution of sodium chloride by dissolving 1 gram of sodium chloride
in 100 grams of water; 5 grams in 100 grams of water; 10 grams in 100
grams of water; and so on. The amount of solute for any given amount of
solvent is defined as the concentration of the solution.

Figure 1.
(Reproduced by permission of

The Gale Group

.)

One way of expressing the concentration of a solution is with the terms
dilute and concentrated. These terms are not very specific. For example, a
solution containing 1 gram of sodium chloride in 100 grams of water and a
second solution containing 2 grams of sodium chloride in 100 grams of
water are both dilute. But the term is appropriate because, at room
temperature, nearly 40 grams of sodium chloride can be dissolved in 100
grams of water. Thus, a solution containing 35 grams of sodium chloride in
100 grams of water could be called a concentrated solution.

Solutions can also be classified as saturated, unsaturated, or
supersaturated. A saturated solution is one that holds all the solute it
possibly can at any given temperature. For example, the solubility of
sodium chloride in water is 37 grams per 100 grams of water. If you make a
solution containing 37 grams of sodium chloride in 100 grams of water, the
solution is said to be saturated; it can't hold any more sodium
chloride.

Any solution containing less than the maximum possible amount of solute is
said to be unsaturated. A solution with 5 grams of sodium chloride (or 10
grams or 20 grams or 30 grams) in 100 grams of water is unsaturated.

Finally, supersaturated solutions are also possible. As bizarre as it
sounds, a supersaturated solution is one that holds more solute than is
possible at some given temperature. The way to make a supersaturated
solution is to make a saturated solution at some higher temperature and
then let the solution cool very carefully.

For example, one could make a saturated solution of sugar in water at
50°C by adding 130 grams of sugar to 100 grams of water. That
solution would be saturated. But then, one could allow the solution to
cool down very slowly. Under those circumstances, it might happen that all
of the sugar would remain in solution even at a temperature of
25°C. But at that temperature, the solubility of sugar is normally
a little over 100 grams per 100 grams of water. Therefore, the cooled
solution would be supersaturated. Supersaturated solutions are normally
very unstable. The slightest movement in the solution, such as simply
shaking it, can cause the excess solute to settle out of the solution.

Sugar solution A has a concentration of 0.9g/mL. Sugar solution B has a concentration of 1.0g/mL. Which of the following is true about these 2 solutions?
A: Both solutions cannot dissolve any more sugar.
B: Solution B is at a higher temperature than solution A.
C: Solution B is sweeter than solution A.
D: More sugar could be added to solution A but not to solution B.